A keyboard is an essential input device of a computer system. The keyboard is equipped with many keys. By depressing a key of the keyboard, a command is inputted into the computer system. Conventionally, many kinds of keys are designed to generate key signals. For example, a mechanical key uses a conductive pin to trigger a circuit switch to generate a key signal. In addition, an optical key generates a key signal according to the result of receiving or obstructing a sensing light.
FIG. 1 is a schematic view illustrating the structure of a conventional optical switch 9. As shown in FIG. 1, a key part 84 is disposed over a circuit board 83. The key part 84 can be pressed by a user. A light emitter 81 and a light receiver 82 for generating a conducting signal are opposed to each other. Moreover, the light emitter 81 and the light receiver 82 are arranged between the key part 84 and the circuit board 83. While the key part 84 is depressed, a shield arm 84a of the key part 84 is inserted into the space between the light emitter 81 and the light receiver 82 to obstruct the optical path between the light emitter 81 and the light receiver 82. Meanwhile, the sensing light from the light emitter 81 cannot be received by the light receiver 82, and thus the optical switch 9 generates a triggering signal.
However, the conventional optical switch 9 still has some drawbacks. As shown in FIG. 1, the light emitter 81, the light receiver 82 and the key part 84 are arranged at the same side of the circuit board 83. That is, all of the key part 84, the light emitter 81 and the light receiver 82 are disposed over the circuit board 83. Moreover, there is a gap between every two adjacent key parts 84. Generally, dust or liquid easily flies or permeates into the gap between the adjacent key parts 84 and stays on the circuit board 83. Because of the interference of dust or liquid, the sensing light from the light emitter 81 cannot be precisely received by the light receiver 82. Under this circumstance, a misjudgment problem occurs, and the precision of the optical switch is deteriorated. Therefore, the conventional optical switch needs to be further improved.